1. Field of the Invention
The present invention relates to a process for removing a solvent from a solution. In particular, although not exclusively, the present invention relates to a process for removing water from an aqueous solution, such as seawater.
2. The Prior Art
Various methods for removing solvents from solutions are known. For example, water may be extracted from seawater by distillation methods such as multi-stage flash distillation. In a multi-stage flash distillation process, seawater is introduced into a series of tubes and heated to an elevated temperature. The heated seawater is then introduced into an evaporation chamber and subjected to a pressure below its vapour pressure. The sudden reduction in pressure causes boiling or flashing to occur. The flashed vapours are separated from the salty residue by condensation on the tubes of the incoming seawater streams. A series of evaporation chambers are employed. Thus, the evaporation or flashing step occurs in multiple stages.
Water may also be separated from seawater by reverse osmosis. In reverse osmosis, seawater is placed on one side of a semi-permeable membrane and subjected to pressures of 5 to 8 MPa. The other side of the membrane is maintained at atmospheric pressure. The resulting pressure differential causes water to flow across the membrane, leaving a salty concentrate on the pressurized side of the membrane. Typically, the semi-permeable membranes have an average pore size of, for example, 1 to 5 Angstroms.
After a period of operation, the pores of the semi-permeable membrane may become obstructed by deposited salts, biological contaminants and suspended particles in the seawater. Thus, higher pressures may be required to maintain the desired level of flow across the membrane. The increased pressure differential may encourage further clogging to occur. Thus, the membranes must be cleaned and replaced at regular intervals, interrupting the continuity of the process and increasing operational costs.
Attempts have been made to reduce the level of clogging of the membrane. For example, the seawater may be pretreated to remove suspended particles and biological matter. Alternatively or additionally, the residual solution on the high-pressure side of the membrane may be discharged at regular intervals to prevent the osmotic pressure from exceeding a predetermined threshold.